1. Field of the Invention
The present invention relates to a liquid developer for electrophotography for use in developing electrostatic latent images in image forming apparatuses such as electrophotographic type copiers, printers and the like.
2. Description of the Related Art
In image formation by an electrophotographic method, an electrostatic latent image is generally formed by the exposure of an image corresponding to a document image on the surface of an electrostatic latent image carrying member such as a photosensitive member or the like, developing said latent image as a visible toner image, and transferring and fixing said toner image on a recording member to produce an image.
Developing methods can be divided into dry-type developing methods and wet-type developing methods.
Dry-type developing methods use colored microparticles (toner) or a carrier having magnetic properties added to a toner. The main components of dry-type toners are normally pigment and a binder resin, with internal and external additives such as charge controller, conduction controllers, plasticizer, separation agents and the like, being added as necessary. Magnetic toners include a magnetic powder such as Fe3O4. In dry-type developing methods, toner is normally charged via contact with a specific surface of the developing device and contact with other toner particles, or in the case of two-component developers containing a carrier, toner is triboelectrically charged through contact of the toner particles and the carrier, as well as ion adsorption, via vacuum ionic discharge, charge injection, electrostatic conduction via electric field, and said toner is transported to the electrostatic latent image region on a latent image carrying member such as a photosensitive member or the like via electrostatic force, mechanical force, magnetic force or the like so as to develop said latent image.
Due to concern that dry-type toners used in dry-type developing will be released into the air via airborne dispersion in the vicinity of the developing device, such toners cannot be made too fine and typically have a mean particle size of several microns to about 10 .mu.m. Dry-type toners do not produce high resolution images due to their relative large particle size.
On the other hand, the most widely used developer in wet-type developing methods is developer including fine colored particles (toner) mainly comprising pigment and binder resin which is dispersed in an electrically insulated medium (carrier fluid). Charging of the toner is accomplished via charging by the binder resin itself, or ion adsorption via a charge controller when such charge controller is added to the colored fine particles and carrier liquid, such that the charged toner accomplishes; development via the principle of electrophoresis.
Toners used in wet-type developing can be used with a mean particle size in the submicron range because there is no concern of airborne dispersion. Therefore, wet-type developers are advantageous inasmuch as they produce high resolution images having excellent halftone characteristics.
In liquid developers used in wet-type developing, important factors include that the binder resin does not affect toner image transfer characteristics, fixing characteristics, coloration, nor affect toner particle chargeability and dispersion characteristics. Chargeability and dispersion characteristics of the toners used in wet-type developers influence developing characteristics and transfer characteristics when using electrostatic transfers.
For example, Japanese Laid-Open Patent Application No. 50-10140 discloses a liquid developer having improved toner image transfer characteristics and fixing characteristics as well as improved toner particle dispersibility and chargeability by using a toner particle comprising a pigment coated by a binder resin comprising semi-soluble graft copolymer produced by copolymerization reaction of an insoluble monomer and a soluble monomer, said toner particles being dispersed in an organic hydrocarbon solvent used as a carrier fluid.
It is difficult, however to control each of the various characteristics such as fixing characteristics, chargeability, and dispersion characteristics to attain the optimum conditions for each characteristic when using a single such binder resin. It is furthermore difficult to maintain such characteristics at their initial levels after repeated image formations wherein concentrated toner replenishment liquid is resupplied over a long period because the individual characteristics cannot be separately maintained by the resupplied toner.
Therefore, it is desirable that transfer characteristics, fixing characteristics, and toner particle chargeability and dispersibility are controlled by separate materials. Specifically, transfer characteristics and fixing characteristics can be stabilized by using as a binder resin a resin which is insoluble in an organic hydrocarbon solvent used as a carrier fluid, and toner particle chargeability and dispersion characteristics can be controlled by adding a separate charge controller. For example, U.S. Pat. No. 5,407,771 discloses a liquid developer using a thermoplastic resin such as ethylene-vinyl acetate copolymer or the like as a binder resin which is insoluble in an organic hydrocarbon solvent.
This binder resin affects light transmittancy of the image after fixing, however. Particularly in the case of full color images formed by overlaying developers of a plurality of colors, the image is darkened when the resin light transmittance is poor, thereby making it difficult to obtain a desired color. The ethylene-vinyl acetate copolymer resin disclosed in the aforesaid U.S. Pat. No. 5,407,771 has poor resin light transmittance in toner images fixed on a recording member, thus making it difficult to obtain suitable color levels when forming color images.
In order to attain stable transfer characteristics and fixing characteristics as well as excellent light transmittance in a liquid developer, it is desirable to select from among thermoplastic resins a well known saturated polyester resin for use as a binder resin in a color toner for use in dry-type developing. Polyester resin not only allows the changing of physical characteristics such as thermal characteristics within a wide range, but also provides beautiful color due to excellent light transmittance when making full color images, and provides excellent adhesion characteristics relative to recording members such as paper and the like due to the toughness of the resin layer after fixing due to its excellent elasticity and flexibility. Liquid developers using such polyester resin as a binder resin are disclosed in Japanese Laid-Open Patent Applications Nos. 7-92742 and 8-220813.
From the perspective of producing high resolution images, wet-type developing methods are finding increased usefulness in high-speed full color printers in place of dry-type printers, and image formation at ever higher speeds is sought.
Regarding this point, the polyester resin disclosed in, for example, Japanese Laid-open Patent Application No. 7-92742 allows only a maximum system speed of about 400 mm/sec when used as a toner binder resin in a liquid developer. This limitation is believed to be due mainly to low developing speed caused by the low amount of charge of the toner particles. Since fixing cannot overtake the system speed, poor fixing readily occurs. This limitation is believed to be caused mainly by excessive fixing, temperature.
When the toner particles have too high a charge, the liquid developer generally becomes unstable, making it difficult to achieve stable image formation over a long period of time.
When the toner fixing temperature is too low, the liquid developer becomes unstable due to the binder resin used to form the toner readily eluting in the carrier liquid, thereby making it difficult to maintain stable image formation over a long time period.